Classmates celebrate 6-year-old after he finishes his last round of chemo

Only 6 years old, John Oliver Zippay has endured a battery of medical procedures that would test even the most stoic adult.

John, nicknamed, "JO," was diagnosed with leukemia at just 3 years old. After his final chemotherapy treatment, his elementary school classmates greeted him back to school with a hero's welcome -- a standing ovation.

"It started with our friend, her son is in JO's class," JO's mom Megan Zippay told ABC News. "She wanted to do a little celebration," when JO returned to school after his last round of chemo.

"JO asked us, 'Maybe they're going to do an announcement that I'm all done with chemo?'" his father, John Zippay, said.

But his classmates had more than an announcement planned. When JO went back to school, his fellow students lined up and stood applauding and cheering for JO as he made his way down the school hallway, beaming.

"I think him coming down the hall and everyone applauding him was a good end," Megan Zippay said.

For JO and his parents, it was the end of a grueling and frightening three years of daily chemotherapy, monthly steroids and doctor visits. JO also had to have a mediport put in his chest.

He was diagnosed with leukemia on Halloween in 2017. His parents had taken him for testing after he had a bruise that wouldn't go away and after their normally active little boy seemed more tired than usual.

"It was a typical day," Megan Zippay said. "He went to school, we went home, he got dressed for trick-or-treating. Then we got a call at four in the morning."

It was the hospital calling with JO's leukemia diagnosis.

"Our entire world just stopped," said John Zippay. "It was a huge shock to us."

The Zippays said they were helped by the outpouring of love and support they received. And when JO made his way down the school hallway celebrated by his classmates, the look on his face "was pure happiness," said Megan Zippay.